This invention relates to improved apparatus for the electrolysis of aqueous solutions of ionizable chemical compounds. More particularly it relates to the production of halogens, e.g., chlorine, and alkali metal hydroxides, e.g., sodium hydroxide, in an electrolytic cell which possesses advantages over previously known cells.
The electrolysis of aqueous solutions of ionizable chemical compounds, particularly brine solutions, in a cell equipped with an anode and a cathode separated by a porous diaphragm is well known in this art. In most instances such cells are operated under conditions such that ionic migration and molecular migration through the porous diaphragm occurs to a substantial degree resulting in the contamination of the cathode liquor with undecomposed electrolyte and of the anode liquor with reaction products of the cathodic material and anodic materials.
Presently, there is a high demand for pure aqueous caustic for use in commercial processes. For example, in the production of rayon or pharmaceuticals, the caustic must contain less than 100 ppm chloride ion, i.e. less than 0.1% chloride ion. In addition, a great many other commercial processes require caustic containing less than 1000 ppm chloride ion, i.e. less than 1.0% chloride ion.
In one prior art process, pure caustic is obtained by electrolysis of brine floating on a mercury cathode to form chlorine gas and a mercury-sodium complex. The complex then is contacted with water to form caustic, mercury and hydrogen gas. The caustic has good purity in that it is free from chloride ion but is impure in that it is contaminated with mercury. This mercury contamination has severly limited the use of the caustic, particularly in the production of pharmaceuticals.
As a substitute from the above-described process, it has been proposed to conduct the electrolysis of brine solutions in a cell wherein the anode and cathode are separated by a fluid-permeable diaphragm such as an asbestos diaphragm to evolve chlorine gas from the anode compartment and caustic from the cathode compartment.
The sodium hydroxide produced by this method is, however, relatively dilute and, because of the fluid permeable nature of the diaphragms used, it is further contaminated with various impurities, such as sodium chloride, sodium chlorate, iron and the like. It is, therefore, necessary to subject the sodium hydroxide product to various evaporation steps and purification steps such as liquid-liquid extraction with ammonia in order to obtain a product which is suitable for many commercial uses, particularly those requiring less than about 1% chloride ion. Moreover, with such electrolytic cells, there is an appreciable back migration of hydroxyl ions from the cathode compartment to the anode compartment which results in the production of hypochlorites which are oxidized to chlorates, with a consequent reduction in chlorine yield and further contamination of the sodium hydroxide. Additionally, depending upon the source of sodium chloride used in making up the brine electrolyte, brine purification systems must frequently be used to eliminate ions such as calcium, that may clog the fluid permeable diaphragms. While these processes are desirable from the standpoint that chloride-free caustic can be obtained, these same processes are undesirable because of the cost of conducting the manipulative steps subsequent to electrolysis.
Attempts have heretofore been made to overcome the aforesaid difficulties in the operation of such diaphragm cells by replacing the fluid permeable asbestos diaphragms with permselective ion exchange membranes. In theory, the use of such membranes which, for example, would permit the passage of only sodium ions from the anode compartment to the cathode compartment, would eliminate the problems of contamination of the sodium hydroxide liquor in the cathode compartment and would prevent the back migration of hydroxyl ions to the anode compartment to thereby reduce hypochlorite and chlorate formation therein. For this purpose, various resins, such as cation exchange resins of the "Amberlite" type, sulfonated copolymers of styrene and divinylbenzene, and the like, have been proposed including those disclosed in U.S. Pat. No. 2,967,807, U.S. Pat. No. 3,390,055 and French Pat. No. 1,510,265. In practice, however, the permselective ion exchange membranes which have been used have generally been found not to be stable to the strong caustic and/or acidic solutions encountered in the cells at operating temperatures above 75 degrees C. so that they have had only a relatively short effective life in the order of only up to about 45 days. Additionally, as the concentration of caustic soda in the catholyte liquor is increased, e.g. above about 200 grams per liter, it has frequently been found that the ion selectivity and chemical compatibility of the membrane decreases, the voltage drop through the membrane becomes unacceptably high and the caustic efficiency of the electrolysis process decreases. These undesirable characteristics of the membrane result in increased costs associated with membrane replacement and cell down-time. It is believed that the poor membrane life observed is caused by chemical attack by chloride ion and/or hypochlorite ion on the membrane or on the functional groups of the membrane. When these functional groups are removed or chemically altered, the selectivity of the membrane for passage therethrough of sodium cation is substantially reduced.
Thus, it has been observed that even membranes known to be resistant to caustic and chloride ion do not exhibit good useful lives in the electroysis cell since the membranes are not resistant to hypochlorite ion. Accordingly, it would be highly desirable to provide an electroysis cell for forming pure caustic from brine which eliminates the need for either a caustic evaporation step or a caustic-extraction treatment step in order to obtain caustic having less than 1% or 0.1% chloride ion. Furthermore, it would be highly desirable to provide such a process having a permselective membrane within the cell wherein the useful life of the membrane is extended significantly beyond the useful life of presently available permselective membranes. Such a process would significantly reduce the cost of producing pure caustic from the standpoint of eliminating manipulative process steps, reducing cell down-time and reducing membrane useage.